The analysis procedure of plane-strain deformation of sands based on bifurcation mechanism is presented through the combination of PIV (particle image velocimetry), double Fourier analysis, and block-diagonalization method. This procedure is applied to the PIV images of the progress of deformation of sand specimens to detect the specific Fourier mode corresponding to the bifurcation mode. The images are decomposed based on block-diagonalization to identify bifurcation modes triggering shear band formation. The procedure is found to be especially suited for the physical comparison of the defomation of experiment and numerical bifurcation analysis